One type of vehicle cooler, which is, for instance, disclosed in EP-A1-0 590 945, comprises a heat exchanger assembly which is made up of, on the one hand, flat fluid conveying tubes, which are juxtaposed to be passed by a first fluid, for instance, liquid circulating through an engine block and, on the other, surface-enlarging means arranged between the tubes and adapted to be passed by a second fluid, e.g. cooling air. Each tube has opposite large faces, to which the surface-enlarging means are applied and which form the primary heat exchanging surfaces of the tube.
In this type of coolers, it is already known to provide the primary surfaces on the inside of the tubes with projections with a view to increasing the heat exchange between the fluids. These projections break up the insulating, laminar boundary layer which otherwise tends to form inside the tube along its primary surfaces, at least at low fluid flow rates. The projections can be elongate, as known from e.g. U.S. Pat. No. 4,470,452, or cylindrical, as known from e.g. U.S. Pat. No. 5,730,213. However, these constructions are not capable of combining a sufficiently high heat exchanging capacity with a sufficiently low pressure drop in the longitudinal direction of the tubes.
An alternative embodiment of fluid conveying tubes is disclosed in a doctor's thesis published in 1997 by Chalmers Institute of Technology entitled “Thermal and hydraulic performance of enhanced rectangular tubes for compact heat exchangers”. Such a tube is schematically shown in a plan view in FIG. 1. The opposite primary surfaces of the tube have transverse ribs 1 in zigzag, i.e. surface structures which each consist of a number of elongate rib elements 2 which are connected to each other in intermediate pointed areas 3. The transverse ribs 1 are alternatingly arranged in the longitudinal direction L of the tube on the opposite primary surfaces of the tube, the ribs 1 (full lines in FIG. 1) arranged on the upper primary surface being transversely offset relative to the ribs 1 (dashed lines in FIG. 1) arranged on the lower primary surface. Seen in the longitudinal direction L of the tube, the succeeding rib elements 2 are arranged alternatingly on the opposite primary surfaces and have a given mutual angle. Thus, the rib elements 2 will direct the flow of the first fluid through the tube to generate a swirling motion about the longitudinal axis of the tube, as schematically shown in the end view in FIG. 2. More specifically, the input flow is divided into a number of parallel partial flows 4 to which a spiral motion is imparted when passing through the tube, each partial flow 4 having an opposite rotation relative to the adjoining partial flows 4. By means of such partial flows, the boundary layer adjacent to the primary surfaces is broken up and a better circulation of fluid is provided between the centre portions and wall portions of the tube. All this results in a potentially high heat exchanging capacity of the tube. It has, however, been found that it is difficult to provide connected ribs in zigzag shape by means of today's manufacturing technique, and therefore there is in practice a gap in the pointed areas 3 between the rib elements 1.
Vehicle coolers with this type of “spiral-flow tubes” have been found to have a high heat exchanging capacity also at relatively small flows through the tubes, which is often desirable, for instance, in vehicle coolers for truck engines with air charging or boosting, since these vehicles can generate large quantities of heat also at low speeds of the engine.
The above construction is, however, in its infancy, and needs to be further developed to optimise its capacity.